A chemical sensor is a device which monitors the concentration of a given chemical species in a liquid or a gas. Chemical sensor devices comprise a sensitive layer, which is sensitive to a particular chemical which is to be detected by the sensor device, and a heater integrated on a semiconductor substrate such as a silicon substrate. The heater increases the temperature of the sensitive layer to increase the sensitivity of the sensor device and is typically required to heat the sensitive layer to temperatures in the range of 25.degree. C. to 600.degree. C. during operation of the sensor device. At these temperatures, there is significant loss of thermal energy through the silicon substrate and therefore such devices suffer from high power consumption.
The high power consumption is a particular problem when the sensor device is required to be powered by a battery. For example, some applications may require battery back-up operation. For such battery powered applications, the power consumption should be around 60 mw at 400.degree. C. in DC mode.
In order to reduce the power consumption in semiconductor chemical sensors, it has been proposed, see for example French Patent Application no. FR-A-2615287, to micromachine the backside of the bulk silicon substrate to form a thin membrane under the active region of the sensor device (i.e. under the heater and sensitive layer). The thin membrane is formed by depositing a solution of Boron Oxide (B.sub.2 O.sub.3) by spin-on onto the silicon substrate and by then diffusing the boron dopant into the substrate. Although this technique reduces the thermal losses to the bulk silicon substrate, since the membrane comprises a silicon layer doped with P+ type material, which material has a high thermal conductivity (35 Wm.sup.-1 K.sup.-1), the thermal losses during use of the sensor and hence power consumption is still too high for low power operation, such as battery back-up operation. For example, at 400.degree. C., the power consumption for such a sensor can be as high as 200 mW in DC mode.
Another technique for reducing thermal loss is described in U.S. Pat. No. 5,545,300. This patent describes using a composite membrane comprising a glass film formed over a silicon nitride film. The reduction in the power consumption with the described technique is limited due to the high thermal conductivity of the silicon nitride film (23 Wm.sup.-1 K.sup.-1) used in the composite membrane. The power consumption of the device disclosed in this patent is close to 110 mW at 400.degree. C. in DC mode, which is still too high for battery operation.
Since two different steps (deposition of silicon nitride film followed by deposition of silicon oxide film) are required to realise the membrane, this patent also suffers from the cost disadvantage mentioned above of having several fabrication steps. A further disadvantage of the process described in this patent is that as the silicon nitride layer is deposited on top of the silicon substrate, such a process step can create dislocations on the silicon surface which is not compatible with integrated circuit (IC) technology. In other words, with such a process, it would not be possible to integrate the control chip on the same substrate as the sensor device.
An article entitled `Reduction of heat loss of silicon membranes by the use of trench-etching techniques` by J. Werno, R. Kersjes, W. Mokwa and H. Vogt, and published on pages 578-581 of Sensors and Actuators A, 41-42 (1994), describes a solution to improve the thermal insulation of single crystal silicon membranes by means of oxide-filled trenches. The power consumption of this solution is 230 mW at 400.degree. C. in DC mode, which is too high for battery operation. In addition, the described trench-etching technique employs three successive process steps (SIMOX/epitaxy/oxide-filled trenches) and is thus an expensive technique to use with respect to the manufacturing costs.
There is therefore a need to provide an improved electronic device which can be operated at low power and a method of forming such an electronic device.